Former Manafort business associate Rick Gates also has been told to turn himself in.

WASHINGTON ― Paul Manafort, onetime campaign chairman to President Donald Trump, surrendered to the FBI Monday. Former Manafort business associate Rick Gates also has been told to turn himself in to federal authorities, multiple news outlets reported on Monday.

Manafort and Gates are expected to be charged in an intensifying investigation into Russian meddling in the United States' 2016 presidential election.

The office of special counsel Robert Mueller, which is conducting the probe, is expected to serve the indictment on Monday. The apparent forthcoming arrests come three days after news broke that a federal grand jury in Washington had filed the first charges stemming from Mueller's investigation.

Trump, who does not have the power to pardon state crimes, has aggressively denied allegations that his team colluded with Russia to win the election.

A former prosecutor and FBI director, Mueller partnered with New York Attorney General Eric Schneiderman in August to look into Manafort's possible financial crimes, including money laundering.

Manafort ran Trump's presidential campaign from June 2016 until his resignation in August of that year, after reports surfaced about his past lobbying work for pro-Russian Ukrainian oligarchs. He retroactively registered as a foreign agent and disclosed more than $17 million in payments for his firm's consulting work in Ukraine, which occurred before he joined Trump's campaign.

"You can get rid of Manafort," said Robby Mook, who was the campaign manager for then-Democratic nominee Hillary Clinton, "but that doesn't end the odd bromance Trump has with [the Russian President]."

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Manafort met with Senate Intelligence Committee investigators in July to discuss a meeting between Trump associates and a Russian lawyer that was held during the election campaign to obtain dirt on Clinton. The next day, FBI agents seized documents and other items from Manafort's house in Virginia without warning.

Investigators have also issued subpoenas seeking testimony from a number of people linked to Manafort throughout the probe.

Trump described the pre-dawn raid of Manafort's home as "pretty tough stuff."

"I've always found Paul Manafort to be a very decent man," the president said at the time. "He's like a lot of other people, probably makes consultant fees from all over the place, who knows, I don't know, but I thought it was pretty tough stuff to wake him up, perhaps his family was there."

Trump fired FBI director James Comey, who was leading the agency's investigation into Russian interference in the election, in May. The following week, the Justice Department appointed Mueller as special counsel to oversee the federal inquiry into the Trump campaign's alleged Russian ties.

"Led by some very bad and conflicted people," Trump tweeted in June, the Russia probe is "the single greatest WITCH HUNT in American political history."